Man-made mounds said oldest in North America

WASHINGTON (AP) Low mounds built about 5,400 years ago on a river plain in what is now Louisiana are the oldest known human-built structures in North America, researchers have determined.
The mounds, at a place called Watson Brake, about 32 km southwest of Monroe, were built by a people who found food in nearby rivers and forests, and occupied the site over hundreds of years, according to Joe W. Saunders of Northeast Louisiana University.
Saunders, lead author of a study to be published Friday in the journal Science, said the people were seasonal hunters and gatherers who ate a lot of fish while living near a river for only a few months at a time.
Bones of catfish, drum and suckers, all common fish found even today in the rivers, were unearthed in the mounds. There was also evidence that the people feasted on turtles, mussels, aquatic snails and small animals.
It has long been believed that such hunters and gatherers lacked the organizational skills to build mounds, which are large humps that required digging up and moving huge amounts of earth, Saunders said.
"These mounds contain hundreds of tons of dirt and gravel," said Saunders. He said there are 11 mounds in a rough circle about 254 meters across. One of the mounds is over 6 meters high. The rest are 1 meter to 3 meters high. Many are connected by excavated ridges.
It was, said Saunders, a major project for people who had to move all of the material by hand.
Chemical dating of materials showed that construction of the mounds started about 5,400 years ago, making them about 1,900 years older than mounds found in Florida and elsewhere in Louisiana.
Just what the primitive people used the mounds for is still not known, said Saunders. He said it is unlikely the mounds were important for defense and they are too far from the river to be used as a refuge from floods.
"The Watson Brake mounds are preceramic," said Saunders. This means that the people living there had no vessels in which to cook.
Instead, he said, they apparently heated rocks and then dashed them with water to make steam. Saunders said his research team found large deposits of fire-cracked gravel, suggesting that the people used red-hot rocks to bake or steam their food.
Also found were large numbers of intact snail shells. This suggests, he said, that the people steamed the snails and then removed the flesh without breaking the shells.